1. Field of the Invention
The present invention relates to processes for improved operation of coal-fired electrical utility boilers and other solid-fuel-fired, high temperature combustion reactors. More particularly, the present invention relates to processes for reducing fouling of solid-fuel-fired boiler internal components, reducing corrosion within such boilers, and reducing undesirable and noxious stack emissions.
2. Description of the Related Art
In coal-fired power generating plants, as well as in other industrial processes involving combustion of coal, the products of the combustion process include compounds that have an adverse influence on boiler operation, or are environmentally undesirable and the discharge of which into the environment is subject to environmental regulations. Such compounds include sulfur oxides (SOx), nitrogen oxides (NOx), and such heavy metals as mercury, arsenic, lead, and cadmium. In order to meet environmental limitations affecting the discharge of the most prevalent sulfur oxide, SO2, into the atmosphere, combustion products from such plants and processes are commonly passed through flue gas desulfurization (FGD) systems. The sulfur oxides contained in such combustion products are thereby converted into less-environmentally-harmful compounds that are either disposed of in landfills, or, when suitably modified or treated, are sold as marketable chemicals.
The treatment of flue gases to capture SO2 is often effected in lime- or limestone-based wet scrubbers, in which lime or limestone slurries are sprayed into the flue gases before they are discharged into the atmosphere. The sulfur oxides are thereby chemically converted into insoluble calcium compounds in the form of calcium sulfites or sulfates. The less environmentally harmful calcium compounds are separated from the scrubber blowdown liquor and either are disposed of in landfills or are converted into marketable gypsum.
Although useful for converting some sulfur oxides, widely-used lime/limestone scrubbers are not very effective in capturing the 1% to 1.5% of the sulfur in the fuel that is transformed during the combustion process into gaseous SO3, which can escape from the scrubber. The SO3 poses operating problems within the boiler itself, in that it leads to corrosion and fouling of low temperature heat exchange surfaces. Additionally, it poses environmental problems in that unless captured or transformed, the SO3 results in a persistent visible plume and the discharge of corrosive and potentially hazardous sulfuric acid fumes. Further complicating the matter, selective catalytic reactors (SCR's), which are available and are installed in such plants to comply with nitrogen oxide emission regulations, essentially cause a doubling of the amount of SO3 that is generated, and consequently the already serious operational and environmental problems caused by the presence of SO3 are magnified.
The SO3 emission problem has been addressed chemically using a variety of alkaline chemicals that are injected into the system at many different points in the flue gas flow path. Lime or limestone injected into the high temperature region of the boiler can be effective in capturing the SO3, but it tends to magnify boiler deposit problems and to increase the quantity of particulates that escape from the precipitators. The adverse impact on the precipitators is also encountered when lime or lime hydrate is injected as powders into the lower temperature region downstream of the SCR's. The precipitator problem can be circumvented by injecting the lime downstream of the precipitator, but an efficient backup dust collector is needed, and most FGD scrubbers are not efficient collectors of fine particulates.
Sodium compounds, such as the bisulfite, carbonate, bicarbonate and carbonate/bicarbonates (Trona) compounds, have also been injected into the cooler regions of the system and are effective in SO3 capture. However, they pose material handling and some deposit problems, and they tend to have poor utilization efficiencies unless they are ground to very fine particle sizes. Relatively coarse particles are prone to formation of an outer sulfate shell, thereby inhibiting utilization of the unreacted chemical inside the shell. Additionally, grinding of such materials is expensive, and it creates storage and handling problems because of the fineness of the particles.
Commercially-available, but relatively expensive, oil-based magnesium additives can be extremely effective in SO3 capture. In that regard, one of the most effective chemical techniques for controlling both ash-related fouling in the boiler, and also the corrosion and emission problems associated with SO3 generated in solid-fueled boilers, is the injection into the upper region of the boiler of oil slurries of MgO or Mg(OH)2. That technology was originally developed for use with oil-fired boilers in which the magnesium-based oil suspension was usually metered into the fuel. It was later applied to coal-fired boilers. The most widely accepted mode of application of such additives today is by injection of slurries of MgO or Mg(OH)2 into the boiler above the burners and just below the region at which a transition from radiant heat transfer to convective heat transfer occurs.
In addition to the oil-based slurries, Mg(OH)2 powders and water-based slurries have also been utilized as fireside additives in boilers, but because of their generally coarser particle size they are less efficient in capturing the SO3. Water slurries of MgO have also been injected through specially modified soot blowers on oil and Kraft-liquor-fired boilers, in which they moderated high temperature deposit problems but had only a nominal impact on SO3-related problems because of an inability to apply the chemicals continuously.
Regulations aimed at controlling mercury emissions of coal-fired boilers have been promulgated by regulatory authorities, and regulations applicable to other toxic metals are anticipated eventually. A considerable amount of research aimed at finding practical techniques for capturing such toxic metals has shown that high surface area solids can capture a significant portion of mercury by adsorption, if the mercury is in an oxidized form rather than an elemental form. Oxidants, either added to or naturally present in the fuel, such as chlorides, can facilitate the oxidation. Although high surface area lime can be effective in mercury capture, it results in operational problems in the form of ash deposits and increased stack emissions. The most widely accepted way to achieve capture has been injection of expensive activated carbons.
It is therefore an object of the present invention to provide processes by which boiler operational and emissions problems can be reduced more economically than is attainable by presently-utilized methods.